1. Field of the Invention
The present invention relates to a wafer processing method of dividing a wafer into a plurality of devices along a plurality of division lines, the devices being formed on the front side of the wafer and partitioned by the division lines.
2. Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a functional layer composed of an insulating film and a functional film is formed on the front side of a substrate such as a silicon substrate, and a plurality of semiconductor devices such as ICs and LSIs are formed like a matrix from this functional layer, thus obtaining a semiconductor wafer having the plural semiconductor devices. The plural semiconductor devices are partitioned by a plurality of crossing division lines formed on the front side of the semiconductor wafer. The semiconductor wafer is divided along these division lines to obtain the individual semiconductor devices as chips.
Further, in an optical device fabrication process, an optical device wafer is provided by forming an optical device layer composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer on the front side of a sapphire substrate or a silicon carbide substrate. The optical device layer is partitioned by a plurality of crossing division lines to define a plurality of regions where a plurality of optical devices such as light emitting diodes and laser diodes are respectively formed. The optical device wafer is cut along the division lines to thereby divide the regions where the optical devices are formed from each other, thus obtaining the individual optical devices as chips.
As a method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the division lines, there has been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer in a subject area to be divided. More specifically, this wafer dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer to thereby continuously form a modified layer inside the wafer along each division line and next applying an external force to the wafer along each division line where the modified layer is formed to be reduced in strength, thereby dividing the wafer into the individual devices (see Japanese Patent No. 3408805, for example).
In the laser processing method for forming the modified layer inside the wafer, the laser beam having a transmission wavelength to the wafer is used. Accordingly, in the case that the wafer is formed of silicon, a laser beam having a wavelength of 1064 nm is used, whereas in the case that the wafer is formed of sapphire, a laser beam having a wavelength of 1064 nm or 532 nm is used.
Further, in the case of forming the modified layer inside the wafer along each division line, it is known that a good modified layer can be formed by making the polarization plane of linearly polarized light of the laser beam parallel to the direction perpendicular to each division line and then applying the laser beam along each division line. Accordingly, in the case of processing an optical device wafer composed of a sapphire substrate and a light emitting layer formed on the front side of the sapphire substrate, the light emitting layer being partitioned by a plurality of crossing division lines to define a plurality of regions where a plurality of optical devices are respectively formed, a good modified layer can be formed inside the sapphire substrate along each division line by applying the laser beam to the wafer from the back side of the sapphire substrate along each division line in the condition where the focal point of the laser beam is set inside the sapphire substrate.